Multiple copy electrostatic hold-down apparatus

ABSTRACT

A multiple copy electrostatic hold-down apparatus includes a unidirectional high voltage generator having one side, usually at ground potential, connected to a pinning applicator while an automatic switching device periodically and alternately applies first one side and then the other side of the generator&#39;&#39;s D.C. output to the back surface of the copyboard. As the applicator is drawn over the outer face of sheets abutted against the copyboard front surface, areas of positive and negative charge are deposited in random distribution on each layer of copy material successively applied to the copyboard whereby electrostatically laminated pinning of such copy material is effected.

Schweriner MULTIPLE COPY ELECTROSTATIC HOLD-DOWN APPARATUS Dec. 9, 1975 Primary Examiner-R. N. Envall, Jr. Assistant ExaminerHarry E. Moose, Jr. Attorney, Agent, or FirmStanley Bilker [57] ABSTRACT A multiple copy electrostatic hold-down apparatus includes a unidirectional high voltage generator having one side, usually at ground potential, connected to a pinning applicator while an automatic switching device periodically and alternately applies first one side and then the other side of the generators D.C. output to the back surface of the copyboard. As the applicator is drawn over the outer face of sheets abutted against the copyboard front surface, areas of positive and negative charge are deposited in random distribution on each layer of copy material successively applied to the copyboard whereby electrostatically laminated pinning of such copy material is effected.

7 Claims, 4 Drawing Figures 75 Inventor: Harold A. Schweriner, Hatfield, Pa. [73] Assignee: The Simco Company, Inc., Lansdale, Pa.

[22] Filed: Nov. 11, 1974 [21] Appl. No.2 522,352

[52] US. Cl. 317/262 E [51] Int. Cl. G03B 27/62 [58] Field of Search 317/262 R, 262 E; 355/113 [56] References Cited I UNITED STATES PATENTS 2,025,123 12/1935 Rahbek 317/262 E 3,195,363 7/1965 Stuetzer 317/262 E 3,359,469 12/1967 Levy et a1. 317/262 E 3,480,364 1 1/1969 Barnett 317/262 E POWER us. Patent De c.9,1975 3,925,712

: r/aza This invention relates to electrostatic holding apparatus and more particularly relates to an electrostatic copyboardor contact printing apparatus in which a, photosensitive sheet and at least one negative areelectrostatically laminated to the copyboard surface preparatory to exposing the photosensitive sheet to light rays transmitted through each of the negatives.

In prior U.S. Pat. No. 3,359,469, there'is shown and described an electrostatic copyboard in which one side of the D.C. output from a unidirectional high voltage generator was applied to the back surface of the copyciently to effect proper pinning were the switching to be delayed until after too many layers had been laid down. On the other hand, if switching were effected too board while one or more sheets of copy material were abutted against the front surface thereof by means of a pinning applicator that provided a conductive path from the outer face of the sheets to the other side of the D.C. generator. The applicator would deposit unipolar charges on such sheets, said charges being opposite in polarity from the DLC. charge applied by the generator to the copyboard. However, as a result of repeated pinning operations, sufficient charge would ultimately deposit on the copyboards front surface to bring that surface to ground potential (i.e., the D.C. level of the low side of the generator) such that the sheets of copy would no longer adhere during subsequent pinning operations. That is, essentially the entire D.C. potential would exist across the insulative plate of the copyboard at this stage while substantially no potential existed across the sheets.

In order to remedy this situation, the aforesaid patent taught that the D.C. voltage applied to the back surface of the copyboard by the generator should be disconnected and said back surface connected to ground. As a consequence of substituting ground for the high voltage previously applied by the generator to the back surface, the front surface of the copyboard would assume by virtue of the potential across the copyboard insulative material a voltage opposite in polarity to thatoriginally applied. Accordingly, sheets could once more be adhered to the copyboards front surface when the applicator would now deposit charges on the outer surfaces of the sheets opposite in polarity to those originally laid down. Thus, additional copyholding operations could be performed with the back surface atground until the copies would no longer adhere again. When the sheets would no longer pin to the grounded copyboard, the D.C. high voltage was again reapplied by connecting the high side of the generator to the copyboard and the process continued by repeating the switching mode of operations.

In the placement of multiple layers of copy on the copyboard surface (as when a plurality of negatives are laminated in overlying disposition upon a photosensitive sheet) much the same conditions occur. That is, after a number of layers have been applied,the pinning applicator will have deposited sufficient charge to bring the upper surface ofthe top layer to ground potential such that the next layer or sheet will not adhere. How- I ever, in pinning multiple copies, it is difficult to deterquate pinningof additional layers after switching.

It is therefore an object of this invention to provide a .multiple copy hold-down apparatus which will enable any number of sheet layers to be electrostatically laminated in secure pinned disposition.

. Anotherobject ofthis invention is to provide an electrostatic copyboard in which a photosensitive sheet and at least one'negative may be electrostatically pinned together in laminated disposition preparatory to exposing the photosensitive sheet to light rays transmitted through each of the negatives.

Other objects of this invention are to provide an improved device of the character described which is easily and economically produced, sturdy in construction, and highly efficient and effective in operation.

The foregoing objectives are achieved by automatically. reversing the DC. output from the unidirectional high voltage generator in its connection to the copyboard wherein there is automatic switching from one side of thepower supply to the other. The switching mode is conducted on a regular periodic basis and rapidly enough so that several intervals of each polarity are applied tothe copyboard during the time it normally takes an operator to.wipe the pinning applicator over a layer of copy held against the copyboards holding surface. In this manner, random areas of positive and negative charge are applied to each layer, such random distributionof the charged areas permitting an adequate overall pinning effect.

With the above and related objects in view, this invention consists in the details of construction and combination of parts as will be more fully understood from the following detailed description. when read in conjunction with the accompanying drawings in which:

.FIG. 1 is'a perspective view, partly broken away and partly schematic, of a multiple copy hold-down apparatus embodying this invention.

FIGS. 2A, 2B and 2C are sectional views demonstrating the'principles of this invention and the charge distribution resulting from the application of the respective layers. I

Referring now in greater detail to the drawings in which similar reference characters refer to similar parts,-ther'e is shown an electrostatic copyboard for holding a photosensitive sheet,generally designated as P, and at least one negative sheet, generally designated as N, in electrostatically pinned laminated disposition thereon. Preparatory to exposing the photosensitive plate A, a unidirectional high voltage generator B havmine when to perform the switching since the 7 amount of charge developed during any one pinning operation depends on the polarizing and dielectric characteristics not only of the insulative material of the copyboard itself but also of the sheets to be pinned.- For example, some of the layers might not becharged suffiing a D.C. output, an applicator C connected to one side of the high voltage generator and adapted to be .wipedover the outermost surfaces of the photosensi- 3,359,469, 3,448,356, 3,473,097 and 3,549,963. For

example, the negative N (or plurality of negatives) would be applied to the top surface of the plate 'A first and the photosensitive sheet P applied last. When exposure is to be made from above, the lamp or lamps 12 could be located over the copyboard in which case the insulative plate A may be a suitable opaque dielectric, such as a phenolic resin, and the electrode 18 may be a copper plate.

In either event, the electrode 18 is connected by way of a lead 21 andcurrent limiting resistor 22 to the high voltage power supply B which is capable of applying from about 1,500 to 25,000 volts D.C., positive or negative, to the surface 16 of insulative plate A. The generator B is a conventional unidirectional electrostatic power supply and is shown in block or schematic form in FIG. 1. For purposes of the instant invention, the

generator B is illustrated as developing a negative output with respect to ground across resistor 24 when switch 26 located on the control panel is closed. A voltage control transformer 28 also located on the control panel permits the voltage output of the generator to be varied as desired within its range.

The applicator C similarly can take a number of forms, and may be a roller, brush or squeegee which is adapted to be moved in contact over the upper surfaces negative D.C. output with respect to ground, a minus voltage will be applied to the electrode 18 during the period the points 32 are closedJWhen the sheet N, or

. negative, is applied to the surface 20 of the insulative plate A while the applicator C is drawn from left to right over-the outer layer of sheet N as shown in FIG. 2A, the sheet will become charged positively. When the points 32 are open, the copyboard will be at ground potential and'no charge will be deposited on the sheet N. Still referring to FIG. 2A, uponclosure of the points and with continued wiping of the applicator C against the outer face of the sheet N, a negative voltage will again be applied to the electrode 18 and plus charges will be deposited on the corresponding zone of the sheet N. At the zone furthest to the right, as shown in FIG. 2A, ground will have been applied to the copyboard since the points 32 are open again, as a consequence of which no charge will be deposited'on the portion of sheet N thereabove.

In FIG. 23, a second sheet N is laid upon the first sheet N in contact with thecopyboard, and the applicator C is drawn over the outer face of the second layer. The second layer N will be charged minus in those areas touched by the applicator during the off or grounded portion of the cycle where the underlying sheet N has a positive charge. The second layer N will be uncharged in those areas touched by the applicator during the of portion of the cycle and in which the underlying sheet N is uncharged. During the'on portion of the cycle, the second layer will be charged plus in those areas touched by the applicator where the underlying sheet N is uncharged but will be uncharged where the underlying sheet N is charged plus.

An example of the charge distribution in three layers is-shown in FIG.'2C wherein a photosensitive sheet P is applied to the laminate of two negatives already pinned to the front surface 20 of the copyboard. During the on portion of the cycle, the applicator deposits plus charges on the third layer (sheet P) in those areas touched thereby where one of the underlying sheets N is charged plus and the other is charged minus or where neither sheet N is charged but deposits no charge in See for example U.S. Pat. Nos. 3,359,469 or 3,844,657.

The essence of the instant invention resides in the automatic switching device D which comprises breaker 1 off the high voltage generator B. It has been found that optimum pinning of plural layers of copy' is accomplished by connecting each mode of the D.C. output for a time interval of about A second to 5 seconds, this being compatible with the speed at which the applicator C is normally drawn over the outer faces of the copy by an operator. In the preferred embodiment of this invention, a period of about 1 second on each mode has been determined to be suitable. The cam 34 is of such a configuration that it closes the breaker points 32 during half of its rotation cycle and opens the points during the other half revolution. The AC. synchonous motor 36 running at 30 RPM will cause the cam 36 to close the breaker points 32 for 1 second and maintain them open for 1 second during each-cycle. When the points are open, the copyboard is automatically coupled to ground through current limiting resistor 22 and bleeder resistor 24.

Referring now to FIG. 2, the invention functions in the following manner: With the generator B having, a

those areas where .one of the underlying sheets is charged plus and the other is uncharged.

During the of portion of the cycle, the applicator C deposits minus charges on the third layer P in areas touched thereby where one of the underlying sheets N is charged plus and the other is uncharged but deposits no charge where both underlying sheets N happen'to be uncharged or where one of the underlying sheets is charged plus and the other is charged minus. Accordingly, random plus and minusareas of charge are deposited in the various layers, the applicator C touching randomly different parts of the successive layers during both the on and of portions of the switching cycle. a

For simplicity in showing the charge distribution in the various layers illustrated in FIGS. 2A, 2B and 2C, it has been assumed that a maximum charge transfer from the applicator occurs at all times when it is in contact with a sheet N or P, it being further assumed that thesheets N and P are all perfect insulators with perfect charge retention. The plus or minus charges deposited by theapplicator C at any given zone are therefore shown to be approximatelyof the same magnitude and approximately equal to the negative of the algebraic sum of the plus and minus charges on the same spot of anyunderlying sheet or. sheets and the electrode 18. In practice however, contact of the applicator with the various surfaces is at best imperfect and variable, and the insulation characteristics of the sheets are often less than perfect and differ from one another. Thus, various levels of charge are deposited and maintained in actual practice, and successive layers exhibit a pattern of plus and minus charges which reflect these imperfect conditions so as to vary somewhat in magnitude and sometimes in polarity from the ideal. This departure from the ideal further randomizes the charge distribution in the various layers N and P and contributes to the desirable effect of having many layers of copy adhere to each other and to the copyboard surface with an effective and approximately equal pinning force.

The random charge distribution not only assures that effective electrostatic pinning will occur in multiple sheets, but also produces a lesser degree of shock when the operator touches the sheets directly. Furthermore, the random plus and minus distribution reduces electrostatic sparking between copy layers and/or the copyholding surface as the layers are removed from the copyboard thereby preventing fogging of photosensitive film due to electrostatic sparks.

While the foregoing description has been directed to the electrostatic pinning of one or more negatives in contact with a photosensitive sheet, it is to be understood that the principles are equally applicable to a plurality of overlays electrostatically laminated to each other preparatory to photographing or the like.

Although this invention has been described in considerable detail, such description is intended as being illustrative rather than limiting since the invention may be variously embodied, and the scope of the invention is to be determined as claimed.

What is claimed is:

1. An electrostatic copyboard for holding a plurality of sheets in laminated disposition thereon comprising:

an insulative plate having opposed surfaces,

a unidirectional high voltage generator having a DC.

output, applicator means adapted for traversal across the outermost face of each of said sheets as such sheet is successively abutted in laminar disposition against one surface of said insulative plate, and

automatic switching means for cyclically altering the manner of connecting said high voltage generator to said applicator means and said insulative plate so that during a first portion of the cycle said applicator means and the other surface of said insulative plate are coupled across opposite sides of the DC. output and then during a second portion of the cycle said applicator means and the other surface of said insulative plate are coupled to the same side of the DC. output whereby discrete areas of positive charges and negative charges are randomly dis tributed through the various sheets being laminated.

2. The invention of claim 1 wherein the applicator means is connected to the ground side of said D.C. outpu 3. The invention of claim 1 wherein said applicator means comprises a conductive member for contact abutment with said sheets.

4. The invention of claim 1 wherein said automatic switching means applies each side of said DC output for time intervals of between A and 5 seconds each.

5. The invention of claim 1 wherein the high voltage side of the DC. output is applied to the other surface of said insulative plate by an extended surface electrode.

6. The invention of claim 5 wherein said automatic switching means is in the input to said high voltage generator.

7. The invention of claim 6 wherein said automatic switching means comprises a cam-actuated switch. 

1. An electrostatic copyboard for holding a plurality of sheets in laminated disposition thereon comprising: an insulative plate having opposed surfaces, a unidirectional high voltage generator having a D.C. output, applicator means adapted for traversal across the outermost face of each of said sheets as such sheet is successively abutted in laminar disposition against one surface of said insulative plate, and automatic switching means for cyclically altering the manner of connecting said high voltage generator to said applicator means and said insulative plate so that during a first portion of the cycle said applicator means and the other surface of said insulative plate are coupled across opposite sides of the D.C. output and then during a second portion of the cycle said applicator means and the other surface of said insulative plate are coupled to the same side of the D.C. output whereby discrete areas of positive charges and negative charges are randomly distributed through the various sheets being laminated.
 2. The invention of claim 1 wherein the applicator means is connected to the ground side of said D.C. output.
 3. The invention of claim 1 wherein said applicator means comprises a conductive member for contact abutment with said sheets.
 4. The invention of claim 1 wherein said automatic switching means applies each side of said D.C. output for time intervals of between 1/4 and 5 seconds each.
 5. The invention of claim 1 wherein the high voltage side of the D.C. output is applied to the other surface of said insulative plate by an extended surface electrode.
 6. The invention of claim 5 wherein said automatic switching means is in the input to said high voltage generator.
 7. The invention of claim 6 wherein said automatic switching means comprises a cam-actuated switch. 